Yuck! shower smells like a septic tank

After running the shower for about 2 min. This oder comes from the shower drain. I cheked under the house and there is a p trap. It is the only bathroom in the house that does this . We had a new septic and leach field put in abouyt 3 years ago. and that is when I beliebe 6the problem started. I thought it was hair and such when I cleaned the drain but the smell came back. or never left. It seems like a venting issue. Steuter valve? another p trap? any thoughts?
Grove

You may be the owner of an incorrectly built shower. Some people seem to think that the laws of gravity go away when they build a shower - they put the shower pan liner flat on the floor, expecting the water that will accumulate there to magically find its way to the drain. Small cracks, poor caulking, and even fully intact grout lets a little moisture under the top layer of the tile. Eventually, that stagnent water does exactly what you are experiencing if it can't find its way to the drain - it starts to smell like a swamp.

After the shower has set for the afternoon and you look at the shower, is the grout on the floor dark still? This would indicate that there is moisture in the pan. In a well built shower, it would be dry after a few hours, even after a marathon shower.